VB.net基础:简单的自定义控件MyPictureBox
首先,以“Windows控件库”建立一个新工程,将一个PictureBox拖到用户控件上。界面就OK了。
然后我们要定义ViewMode。先建立一个enum:
Public Enum VMode
FitSize = 0
StretchImage = 1
TrueSize = 2
End Enum
然后建立一个变量存放ViewMode属性:
Private vViewMode As VMode = VMode.TrueSize
当ViewMode变化的时候,控件有必要向外界发出一个事件:
Public Event ViewModeChanged(ByVal Mode As VMode)
现在可以写ViewMode属性了:
Public Property ViewMode() As VMode
Get
Return vViewMode
End Get
Set(ByVal Value As VMode)
Dim changed As Boolean = False
If Value <> vViewMode Then changed = True
vViewMode = Value
If changed Then ShowPic()
RaiseEvent ViewModeChanged(Value)
End Set
End Property
这里,ShowPic()是尚未定义的方法,作用就是显示图片。
另外,既然是MyPictureBox,那就要有PictureBox的样。所以要加上一个Image属性:
Public Property Image() As Image
Get
Return PictureBox1.Image
End Get
Set(ByVal Value As Image)
If Value Is Nothing Then '判断一下Value是不是空值。
PictureBox1.BorderStyle = BorderStyle.None
Exit Property
Else
PictureBox1.BorderStyle = BorderStyle.FixedSingle
End If
Value = New Bitmap(Value)
PictureBox1.SizeMode = PictureBoxSizeMode.StretchImage
PictureBox1.Image = Value
ImageRate = Value.Width / Value.Height
ShowPic()
End Set
End Property
这里ImageRate是为了避免重复计算的开销的定义的一个变量,用来存储图像的宽高比,类型是Single。可以在前面加上它的定义。
现在我们来写ShowPic。为了代码结构分明,ShowPic内容很简单:
Private Sub ShowPic()
If PictureBox1.Image Is Nothing Then Exit Sub
If PictureBox1.Visible = False Then PictureBox1.Visible = True
ResizeImage()
End Sub
之所以要把ResizeImage单独写开来是因为这部分还有其他地方要重用。ResizeImage的作用是根据当前的ViewMode值,来改变PictureBox1的SizeMode属性,并决定是否对图片进行缩放:
Private Sub ResizeImage()
If PictureBox1.Image Is Nothing Then Exit Sub
If vViewMode = VMode.FitSize Then
If PictureBox1.Image.Width > Me.Width Or PictureBox1.Image.Height > Me.Height Then
StretchImage()
Else
If Not PictureBox1.SizeMode = PictureBoxSizeMode.AutoSize Then PictureBox1.SizeMode = PictureBoxSizeMode.AutoSize
End If
ElseIf vViewMode = VMode.StretchImage Then
StretchImage()
Else
If Not PictureBox1.SizeMode = PictureBoxSizeMode.AutoSize Then PictureBox1.SizeMode = PictureBoxSizeMode.AutoSize
End If
LocateImage()
End Sub
这一段判断有些复杂,其实应该可以写得更好一点的。StretchImage和LocateImage的作用从名字上大概就能看出来了。先看看StretchImage。它是真正缩放图片的方法。其实也不复杂:
Private Sub StretchImage()
PictureBox1.SizeMode = PictureBoxSizeMode.StretchImage
If MeRate < ImageRate Then
PictureBox1.Width = Me.Width
PictureBox1.Height = PictureBox1.Width / ImageRate
Else
PictureBox1.Height = Me.Height